Microsoft Patches Azure AD Flaw That Hackers Could Use for Brute-Force Attacks

Microsoft announced that it would soon fix a bug that could allow bad actors to use brute-force attacks to gain access to its Azure Active Directory service.

What is the Issue?

SecureWorks reported the issue to Microsoft on June 29. However, the bug was also discovered and reported to the tech giant last year by threat researcher Dirk-Jan Mollema. SecureWorks privately noted the flaw back in September and then publicly posted about it on Wednesday in a blog post.

Data Breach Today reports that,

“SecureWorks says there’s a flaw in the protocol that is used as part of Azure Active Directory’s Seamless Single Sign-On feature.”

SecureWorks says,

“This flaw allows threat actors to perform single-factor brute-force attacks against Azure Active Directory (Azure AD) without generating sign-in events in the targeted organization’s tenant.”

The most alarming aspect is that hackers could execute the attack remotely. Although the cybercriminals might gain access using brute force, if the customer has MFA (multi-factor authentication) turned on, they would not be able to take over the account. Unfortunately, however, many customers do not have MFA turned on.

Azure AD confirms when the correct password is used, so even if the bad actor can’t get into the account, if the customer reused the same password on other accounts, it could be used in credential stuffing attacks on other websites.

How Did Microsoft Respond?

Data Breach Today divulges that initially, “Microsoft told SecureWorks that Azure AD was working by design.” However, they have decided to make two changes to secure the login process further and prevent brute-force attacks on the Azure AD platform. The timing of the fix is essential since a proof-of-concept has already emerged.

Data Breach Today explains the technical details of an attack,

“The vulnerability is wrapped into a complex authentication flow using the Kerberos protocol. The flow is designed to allow someone who is logged into a domain-joined computer to automatically be logged into Azure AD, which is the cloud version of Active Directory. SecureWorks published a diagram, shown below, that shows the authentication flow for the windows transport endpoint - which is not vulnerable - as an example.”

“A successful attack relies on taking advantage of an endpoint called “usernamemixed,” according to SecureWorks’ advisory. The existence of the endpoint was publicly noted two years ago in a  blog post  by  Joosua Santasalo, a security researcher and Azure expert who is based in Finland.”

“Syynimaa says Microsoft plans to change Azure AD so that the “usernamemixed” endpoint is off by default, ensuring those that don’t need it won’t be put at inadvertent risk. Microsoft also says it will make sure even if that endpoint is turned on, login attempts will be logged, Syynimaa says. Researchers had reported differing outcomes as to whether those attempts were logged now.”

What is a Brute Force Attack?

A brute-force attack is when hackers use special software that tries hundreds and thousands of different combinations of usernames and passwords to try to log into secure websites. Most often, attackers use dictionary brute-force attacks where they rotate through a list of commonly used passwords and popular words. However, more sophisticated techniques use complex algorithms to try longer, stronger password combinations, and often they can crack a good password in minutes.

Credential stuffing is another type of brute-force attack where stolen credentials from data breaches are used against other websites and break into user accounts. Unfortunately, many people reuse the same password on all their accounts, making them vulnerable to this type of attack.

A reverse brute-force attack uses a known password and tries to decipher and pair a username to go with it.